Add a way to modify multiple OAs at once in custom scenario creation

The steps for this are probably something like:

[x] change the type of clickedId to a ~~Set<string>~~ I'm just using string[] because Svelte's reactivity doesn't play well with objects, including Sets.
[x] add a flag to determine if the user is in the modify-OAs phase of custom scenario building. ~~We can just use a prop to pass this between components.~~ I made it a store.
[x] If said flag is false, preserve the behaviour below (making sure to never assign more than one element to clickedId)

[x] If the flag is set but shift key was not held down, preserve the same behaviour
- shift key can be obtained from e.originalEvent.shiftKey where e is the MapLibre event
- Additionally, I had to disable the map's boxZoom inside the custom scenario creation so that shift-click events were recorded. See https://github.com/mapbox/mapbox-gl-js/issues/6971.
[x] If the flag is set AND shift key was held down, make it such that clicking an OA adds it to clickedId, clicking it again removes it
[x] If the flag is set AND shift key was held down, turn the defaultPrevented behaviour off as it would be very annoying to click outside an OA during the custom scenario creation and lose all the OAs gotten so far Rejected for UI intuitiveness reasons. Most programmes which allow shift-clicking also deselect everything when you click outside.

Adjust box in left sidebar accordingly to deal with multiple OAs being changed at once.
- [x] Make it so that changing the sliders when multiple OAs are selected updates all OAs
- [x] Make the sliders display the correct "aggregate" values when selecting multiple OAs

Open UX question(s):

[x] Is the current usage of setFeatureState good enough? Currently, when clicking on multiple OAs, this will lead to all borders being highlighted (including inner borders). That's probably ok(?). (Update: This is how it is right now - probably OK, pending discussion with the team.)
[x] Should the click popup be disabled when modifying OAs? There is already a hover popup. Lean towards yes, but should test it out. (Update: Click popup is removed in custom scenario creation. Doesn't seem to be a huge loss, and makes the maplibre state management code less painful.)

Urban-Analytics-Technology-Platform / demoland-web